The present invention relates to an optical pulse generation system and, more particularly, to a method and device for outputting optical pulses having high duty ratio, and an optical sampling pulse generation apparatus using the device.
Conventionally, as a device for generating high-duty optical pulses which have a repetition frequency ranging from several GHz to 10 GHz used in an optical communication system and a small pulse width, an optical pulse generation device using an electroabsorption optical modulator has been proposed, as shown in FIG. 14 (Jpn. Pat. Appln. KOKAI Publication No. 5-283804).
In this optical pulse generation device, for example, light a, which is a laser beam output from a single-wavelength light source 1 such as a laser beam source or the like, and has a continuous, single wavelength, becomes incident on the light entrance surface of an electroabsorption optical modulator 2.
The modulator electrode of this electroabsorption optical modulator 2 receives a pulse modulation signal c, which is obtained by adding a negative DC voltage Va output from a DC voltage source 4 to a sine-wave signal b output from a sine-wave generator 3.
The electroabsorption optical modulator 2 outputs, from its light exit surface, optical pulses d obtained by modulating the light a, which becomes incident on its light entrance surface and has a continuous, single wavelength, by the pulse modulation signal c input to its modulator electrode.
The electroabsorption optical modulator 2 has light absorption characteristics A shown in FIG. 15 with respect to a negatively applied DC voltage V.
A DC voltage V is plotted in a linear scale on the abscissa of the light absorption characteristics A, while the light intensity to be output is plotted in a logarithmic (decibel: dB) scale along the ordinate.
Hence, when the sine-wave signal b is applied to the modulator electrode of this electroabsorption optical modulator 2 at a bias point that has shifted in the negative direction by the negative DC voltage Va, as shown in FIG. 16A, optical pulses d shown in FIG. 16B are output from the light exit surface of this electroabsorption optical modulator 2.
More specifically, as the optical pulses d, pulses 5 that define a waveform in which minus portions of the sine-wave signal b are compressed and its plus portions are amplified in a linear scale appear at periods Ta of the sine-wave signal b, as shown in FIG. 16B.
The pulse width Ts of each pulse 5 that forms the optical pulses d output from the light exit surface of the electroabsorption optical modulator 2, i.e., from the optical pulse generation device is expressed by the width of a portion 3 dB below the top of that pulse 5.
The pulse width Ts of the portion 3 dB below is greatly smaller than the period Ta of the sine-wave signal b.
Hence, using the pulse modulation signal c obtained by adding the negative DC voltage Va to the sine-wave signal b, the high-duty optical pulses d having the pulse width Ts greatly smaller than the repetition period Ta can be obtained.
In recent years, in an optical communication system, the transfer rate of optical signals is increasing, and an optical pulse generation device capable of generating optical pulses d having a pulse width Ts still smaller than the repetition period Ta is required.
Such requirement is also adamant in an optical sampling pulse generation device that outputs optical sampling pulses used to sample light to be measured, which becomes incident on a light sampling unit in a light sampling waveform measurement apparatus.
Hence, since the pulse width Ts of the optical pulses generated by the optical pulse generation device shown in FIG. 14 depends on the period Ta of the sine-wave signal b, i.e., a frequency fA of the sine-wave signal, a pulse width still smaller than the repetition period Ta cannot be obtained.
To combat this problem, an optical pulse generation device shown in FIG. 17 has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 9-133901).
This optical pulse generation device shown in FIG. 17 comprises a plurality of sine-wave generators 3a and 3b for outputting sine-wave signals b1 and b2 (see FIG. 18B; only b1 is illustrated) having different frequencies with respect to a sine-wave generator 3 for outputting a fundamental sine-wave signal b (see FIG. 18A having a frequency fA.
The sine-Wave signals b1 and b2 are respectively delayed a predetermined amount by delay circuits 6a and 6b, and the delayed signals are added to the fundamental sine-wave signal b, thus obtaining a pulse modulation signal c having sharp peaks, as shown in FIG. 18C.
By applying the pulse modulation signal c having sharp peaks to the modulator electrode of an electroabsorption optical modulator 2, the pulse width Ts alone can be shortened without changing the repetition period Ta of optical pulses d output from the light exit surface of this electroabsorption optical modulator 2.
Furthermore, conventionally, an optical pulse generation device using a rectangular wave signal shown in FIG. 19 has been proposed (Jpn. Pat. Appln. KOKAI Publication No. 5-283804).
In the optical pulse generation device shown in FIG. 19, light a output from a single-wavelength light source 1 is modulated into optical pulses d1 by a first electroabsorption optical modulator 2a, and is then modulated into optical pulses d2 by a second electroabsorption optical modulator 2b. 
A rectangular wave signal e1 (FIG. 20A) output from a rectangular wave generator 7 is applied to the first electroabsorption optical modulator 2a. 
On the other hand, a rectangular signal e2 (FIG. 20B), which is obtained by delaying the rectangular wave signal e1 output from the rectangular wave generator 7 a predetermined period of time by a delay circuit 8, is applied to the second electroabsorption optical modulator 2b. 
As a result, optical pulses d2 output from the second electroabsorption optical modulator 2b include pulses 5 having a pulse width Ts corresponding to the overlapping time between the rectangular wave signals e1 and e2, as shown in FIG. 20C.
Hence, when a short overlapping time between the rectangular wave signals e1 and e2 is set by adjusting the delay time of the delay circuit 8, as shown in FIGS. 20A, 20B, and 20C, only the pulse width TS can be shortened without changing the repetition period Ta of the output optic al pulses d2.
However, the optical pulse generation devices shown in FIGS. 17 and 19 still suffer the following problems.
In the optical pulse generation device shown in FIG. 17, in order to obtain the pulse modulation signal c having sharp peaks shown in FIG. 18C, the delay amounts in delay circuits 6a and 6b must be set with high precision while synchronizing sine-wave generators 3, 3a, and 3b. 
In this case, since the frequency of the sine-wave signal b is as very high as several GHz to 10 GHz, complicated setting adjustment is required to set the delay amounts with high precision, and the delay amount may vary soon even after they are set with high precision.
Furthermore, the optical pulse generation device shown in FIG. 17 requires a large number of sine-wave generators 3, 3a, and 3b, and delay circuits 6a and 6b, resulting in a complicated circuit arrangement.
In the optical pulse generation device shown in FIG. 19, since the repetition frequency of the rectangular wave signal e1 output from the rectangular wave generator 7 is as very high as several GHz to 10 GHz, jitter of around 1 ps is produced at the leading and trailing edges of the rectangular wave.
As a consequence, jitter of around 1 ps is produced in the optical pulses d2 output from this optical pulse generation device.
The jitter of around 1 ps is not negligible for optical pulses which are required to have a pulse width Ts of 3 to 4 ps.
Consequently, in this pulse generation device, decreasing the pulse width Ts of the output optical pulses d2 by setting a short overlapping time between the rectangular wave signals e1 and e2 is limited.
The present invention has been made in consideration of the above situation, and has as its object to provide a method and device for generating optical pulses, which can form a sharp peak waveform in a pulse modulation signal to be applied to an electroabsorption optical modulator by a simple circuit arrangement, and can still shorten only the pulse width compared to the repetition period of output optical pulses, and an optical sampling pulse generation apparatus using the device.
In order to achieve the above object, according to the present invention,
(1) there is provided an optical pulse generation method in which a pulse modulation signal is applied to an electroabsorption optical modulator while continuous, single-wavelength light is launched into the electroabsorption optical modulator so as to modulate the incoming single-wavelength light by the pulse modulation signal and to output the modulated light as optical pulses, comprising the steps of:
generating an electrical signal having a sine waveform;
extracting only a waveform not less than a predetermined DC voltage from the sine waveform of the electrical signal; and
adding a negative DC voltage to the extracted waveform, and applying the sum signal to the electroabsorption optical modulator as the pulse modulation signal.
In order to achieve the above object, according to the present invention,
(2) there is provided an optical pulse generation device comprising:
a single-wavelength light source for outputting continuous, single-wavelength light;
an electroabsorption optical modulator for receiving the single-wavelength light output from the single-wavelength light source, modulating the incoming single-wavelength light according to an externally applied pulse modulation signal, and outputting the modulated light as optical pulses;
a sine-wave voltage generator for generating an electrical signal having a sine waveform;
a nonlinear circuit for extracting only a waveform not less than a predetermined DC voltage from the sine waveform of the electrical signal generated by the sine-wave voltage generator; and
a DC voltage source for adding a negative DC voltage to the extracted waveform by the nonlinear circuit, and applying the sum signal to the electroabsorption optical modulator as the pulse modulation signal.
In order to achieve the above object, according to the present invention,
(3) there is provided an optical pulse generation device according to (2), wherein the nonlinear circuit comprises:
a half-wave rectification circuit for outputting a half-wave rectified signal of the electrical signal having the sine waveform, which is output from the sine-wave voltage generator; and
a voltage amplifier for voltage-amplifying the half-wave rectified signal output from the half-wave rectification circuit.
In order to achieve the above object, according to the present invention,
(4) there is provided an optical pulse generation device according to (2), wherein the nonlinear circuit comprises a voltage amplifier, an operation point of which can be shifted, and voltage-amplifies the electrical signal having the sine waveform output from the sine-wave voltage generator while the operation point is shifted in a negative direction.
In order to achieve the above object, according to the present invention,
(5) there is provided an optical pulse generation device comprising:
a single-wavelength light source for outputting continuous, single-wavelength light;
a first electroabsorption optical modulator for receiving the single-wavelength light output from the single-wavelength light source, modulating the incoming single-wavelength light according to an externally applied pulse modulation signal, and outputting the modulated light as optical pulses;
a first modulation signal generation circuit comprising:
a) a sine-wave voltage generator for generating an electrical signal having a sine waveform,
b) a nonlinear circuit for extracting only a waveform not less than a predetermined DC voltage from the sine waveform of the electrical signal generated by the sine-wave voltage generator, and
c) a DC voltage source for adding a negative DC voltage to the extracted waveform by the nonlinear circuit, and applying the sum signal to the first electroabsorption optical modulator as the pulse modulation signal;
a second electroabsorption optical modulator for receiving the optical pulses output from the first electroabsorption optical modulator, modulating the incoming optical pulses according to an externally applied pulse modulation signal, and outputting the modulated optical pulses as final optical pulses; and
a second modulation signal generation circuit comprising:
a) a DC voltage source for outputting a negative DC voltage,
b) a rectangular wave signal generation circuit for outputting a rectangular wave signal, and
c) a sync adjustment circuit,
the second modulation signal generation circuit synchronously adjusting the rectangular wave signal output from the rectangular wave signal generation circuit by the sync adjustment circuit to make individual rectangular waves of the rectangular wave signals include optical pulses once every predetermined number of pulses, adding the negative DC voltage output from the DC voltage source to the adjusted rectangular wave signal, and applying the sum signal to the second electroabsorption optical modulator as the pulse modulation signal.
In order to achieve the above object, according to the present invention,
(6) there is provided an optical pulse generation device comprising:
a single-wavelength light source for outputting continuous, single-wavelength light;
an electroabsorption optical modulator for receiving the single-wavelength light output from the single-wavelength light source, modulating the incoming single-wavelength light according to an externally applied pulse modulation signal, and outputting the modulated light as optical pulses;
a modulation signal generation circuit comprising:
a) a sine-wave voltage generator for generating an electrical signal having a sine waveform,
b) a nonlinear circuit for extracting only a waveform not less than a predetermined DC voltage from the sine waveform of the electrical signal generated by the sine-wave voltage generator, and
c) a DC voltage source for adding a negative DC voltage to the extracted waveform by the nonlinear circuit, and applying the sum signal to the electroabsorption optical modulator as the pulse modulation signal; and
a dispersion decreasing fiber for receiving the optical pulses output from the electroabsorption optical modulator at one end, and outputting the incoming optical pulses as final optical pulses from the other end.
In order to achieve the above object, according to the present invention,
(7) there is provided an optical sampling pulse generation device for outputting optical sampling pulses used to sample light to be measured, which enters a light sampling unit in a light sampling waveform measurement apparatus, comprising:
a single-wavelength light source for outputting continuous, single-wavelength light;
a first electroabsorption optical modulator for receiving the single-wavelength light output from the single-wavelength light source, modulating the incoming single-wavelength light according to an externally applied pulse modulation signal, and outputting the modulated light as optical pulses;
a first modulation signal generation circuit comprising:
a) a nonlinear circuit for extracting only a waveform not less than a predetermined DC voltage from an electrical signal having a sine waveform, and
b) a DC voltage source for adding a negative DC voltage to the extracted waveform by the nonlinear circuit, and applying the sum signal to the first electroabsorption optical modulator as the pulse modulation signal;
a second electroabsorption optical modulator for receiving the optical pulses output from the first electroabsorption optical modulator, modulating the incoming optical pulses according to an externally applied pulse modulation signal, and outputting the modulated optical pulses as final optical pulses; and
a second modulation signal generation circuit comprising:
a) a DC voltage source for outputting a negative DC voltage,
b) a rectangular wave signal generation circuit for outputting a rectangular wave signal, and
c) a sync adjustment circuit,
the second modulation signal generation circuit synchronously adjusting the rectangular wave signal output from the rectangular wave signal generation circuit by the sync adjustment circuit to make individual rectangular waves of the rectangular wave signals include optical pulses once every predetermined number of pulses, adding the negative DC voltage output from the DC voltage source to the adjusted rectangular wave signal, and applying the sum signal to the second electroabsorption optical modulator as the pulse modulation signal;
a dispersion decreasing fiber for receiving the optical pulses output from the second electroabsorption optical modulator at one end, and outputting the incoming optical pulses as final optical pulses from the other end;
a timing clock generation circuit for supplying a first sampling clock signal which is delayed a predetermined time from a clock signal based on a repetition period of the light to be measured to the nonlinear circuit of the first modulation signal generation circuit as the electrical signal having the sine waveform; and
a frequency divider for supplying a second sampling clock signal which is obtained by frequency-dividing the first sampling clock signal from the timing clock generation circuit at a predetermined ratio to the sync adjustment circuit of the second modulation signal generation circuit.
Invention above is applied to an optical pulse generation method in which a pulse modulation signal is applied to an electroabsorption optical modulator while continuous, single-wavelength light becomes incident on the electroabsorption optical modulator to modulate the incoming single-wavelength light by the pulse modulation signal, thus outputting the modulated light as optical pulses.
In order to achieve the above object, in invention (1), an electrical signal having a sine waveform is generated, only a waveform equal to or higher than a predetermined DC voltage is extracted from the sine waveform of the electrical signal, a negative DC voltage is added to the extracted waveform, and the sum signal is then applied to the electroabsorption optical modulator as the pulse modulation signal.
The optical pulse generation device according to invention (2) above comprises a single-wavelength light source for outputting continuous, single-wavelength light, an electroabsorption optical modulator for receiving the single-wavelength light output from the single-wavelength light source, modulating the incoming single-wavelength light on the basis of an externally applied pulse modulation signal, and outputting the modulated light as optical pulses, a sine-wave voltage generator for generating an electrical signal having a sine waveform, a nonlinear circuit for extracting only a waveform not less than a predetermined DC voltage from the sine waveform of the electrical signal generated by the sine-wave voltage generator, and a DC voltage source for adding a negative DC voltage to the extracted waveform by the nonlinear circuit, and applying the sum signal to the electroabsorption optical modulator as the pulse modulation signal.
In the optical pulse generation device with this arrangement, the electrical signal output from the sine-wave voltage generator is rectified and amplified by extracting only the waveform equal to or higher than the predetermined DC voltage from its sine waveform.
That is, the electrical signal is sliced at an arbitrary signal level including the waveform center of half-wave rectification in a sine-wave signal waveform, and the sliced waveform portion is amplified.
Hence, substantially the same effect as in the optical pulse generation method of invention (1) can be obtained.
In the optical pulse generation method and device with the above arrangement, the pulse modulation signal applied to the electroabsorption optical modulator is the sum signal obtained by extracting only the waveform equal to or higher than the predetermined DC voltage from the sine waveform of the electrical signal, and adding the negative DC voltage to the extracted waveform
In the optical pulse generation device according to invention (3) above, in invention (2), the nonlinear circuit comprises a half-wave rectification circuit for outputting a half-wave rectified signal of the electrical signal having the sine waveform, which is output from the sine-wave voltage generator, and a voltage amplifier for voltage-amplifying the half-wave rectified signal output from the half-wave rectification circuit.
In the optical pulse generation device with this arrangement, the pulse modulation signal applied to the electroabsorption optical modulator is the sum signal obtained by amplifying the half-wave rectified signal by the amplifier, and adding the negative DC voltage to the amplified half-wave rectified signal.
In this case, the voltage value of the pulse modulation signal applied to the electroabsorption optical modulator inevitably has an upper limit value.
Since the bias point in light absorption characteristics A shown in FIG. 15 is also limited, the amplitude value (Pxe2x80x94P) of the amplified half-wave rectified signal to be added to the negative DC voltage VA is limited.
The waveforms of the respective signals when the amplitude value (Pxe2x80x94P) of the half-wave rectified signal is equal to that of the sine-wave signal before half-wave rectification are compared.
Of these signals, the half-wave rectified signal waveform is obtained by extracting only positive portions of the sine-wave signal waveform.
Hence, when the amplitude values (Pxe2x80x94P) are equal to each other, the half-wave rectified signal waveform is elongated in the vertical direction compared to the sine-wave signal waveform.
As a result, the half-wave rectified signal waveform defines a sharp peak waveform that has larger changes in voltage per unit time than the sine-wave signal waveform.
Hence, the pulse width Ts of modulated optical pulses output from the electroabsorption optical modulator can be greatly smaller than that of conventional optical pulses modulated using the sine-wave signal.
Note that the repetition period Ta of optical pulses is determined by the frequency fA of the sine-wave signal before half-wave rectification, and is not shortened.
Therefore, only the pulse width Ts can be further shortened compared to the repetition period Ta of optical pulses output from the electroabsorption optical modulator.
In the optical pulse generation device with this arrangement, an electrical signal output from the sine-wave voltage generator is rectified and amplified by extracting only a waveform equal to or higher than a predetermined DC voltage from the sine waveform of that electrical signal. That is, the electrical signal is sliced at an arbitrary signal level including the waveform center of half-wave rectification of claim 2 in a sine-wave signal waveform, and the sliced waveform portion is amplified. Hence, substantially the same effect as in the optical pulse generation method of invention can be obtained.
In the optical pulse generation device according to invention (4) above, in invention (2), the nonlinear circuit comprises a voltage amplifier, an operation point of which can be shifted, and voltage-amplifies the electrical signal having the sine waveform output from the sine-wave voltage generator while the operation point is shifted in a negative direction.
That is, this optical pulse generation device comprises a single-wavelength light source for outputting continuous, single-wavelength light, an electroabsorption optical modulator for receiving the single-wavelength light output from the single-wavelength light source, modulating the incoming single-wavelength light on the basis of an externally applied pulse modulation signal, and outputting the modulated light as an optical pulse, and a modulation signal generation circuit which comprises a DC voltage source for outputting a negative DC voltage, a sine-wave generation circuit for outputting a sine-wave signal, and a voltage amplifier whose operation point can be shifted, amplifies the sine-wave signal by the voltage amplifier while the operation point is shifted in the negative direction, adds the negative DC voltage to the amplified sine-wave signal, and applies the sum signal to the electroabsorption optical modulator as the pulse modulation signal.
The voltage amplifier whose operation point can be shifted is assembled in the optical pulse generation device with this arrangement. The sine-wave signal is amplified by that voltage amplifier while the operation point is shifted in the negative direction.
Hence, the amplified sine-wave signal has a waveform obtained by cutting off some minus components.
As a result, the amplified sine-wave signal is approximated to the amplified half-wave rectified signal.
At this time, when the amplitude value (Pxe2x80x94P) of the amplified signal is constant, since the amplified sine-wave signal has a sharp peak waveform having a larger change in voltage per unit time, as described above, nearly the same effect as in the above inventions can be obtained.
An optical pulse generation device according to invention (5) comprises a single-wavelength light source for outputting continuous, single-wavelength light, a first electroabsorption optical modulator for receiving the single-wavelength light output from the single-wavelength light source, modulating the incoming single-wavelength light on the basis of an externally applied pulse modulation signal, and outputting the modulated light as an optical pulse, a first modulation signal generation circuit which substantially comprises a DC voltage source for outputting a negative DC voltage, a half-wave rectification circuit for outputting a half-wave rectified signal, and a voltage amplifier, amplifies the half-wave rectified signal by the voltage amplifier, adds the negative DC voltage to the amplified half-wave rectified signal, and applies the sum signal to the first electroabsorption optical modulator as the pulse modulation signal, a second electroabsorption optical modulator for receiving optical pulses output from the first electroabsorption optical modulator, modulating the incoming optical pulses on the basis of an externally applied pulse modulation signal, and outputting the modulated optical pulses as final optical pulses, and a second modulation signal generation circuit which comprises a DC voltage source for outputting a negative DC voltage, a rectangular wave signal generation circuit for outputting a rectangular wave signal, and a sync adjustment circuit, synchronously adjusts the rectangular wave signal output from the rectangular wave signal generation circuit using the sync adjustment circuit so that individual rectangular waves of the rectangular wave signal include optical pulses once every predetermined number of pulses, adds the negative DC voltage to the adjusted rectangular wave signal, and applies the sum signal to the second electroabsorption optical modulator as the pulse modulation signal.
In the optical pulse generation device with this arrangement, light output from the single-wavelength light source is modulated by the first electroabsorption optical modulator to obtain optical pulses, and these optical pulses are further modulated by the second electroabsorption optical modulator to obtain final optical pulses.
Since the first modulation signal generation circuit having the same arrangement as that of the modulation signal generation circuit of claim 2 is connected to the first electroabsorption optical modulator, optical pulses output from the first electroabsorption optical modulator have a small pulse width Ts.
Since the pulse modulation signal of the rectangular wave signal, that includes optical pulses once every predetermined number of pulses is applied to the second electroabsorption optical modulator, the optical pulses output from the first electroabsorption optical modulator are output once every predetermined number of pulses by decimation.
As a consequence, the repetition period Ta of pulses is prolonged. However, since the pulse width Ts of each pulse of the optical pulses remains the same, the pulse width Ts can be still shortened compared to the pulse repetition period Ta.
An optical pulse generation device according to invention (6) comprises a single-wavelength light source for outputting continuous, single-wavelength light, an electroabsorption optical modulator for receiving the single-wavelength light output from the single-wavelength light source, modulating the incoming single-wavelength light on the basis of an externally applied pulse modulation signal, and outputting the modulated light as an optical pulse, a modulation signal generation circuit which substantially comprises a DC voltage source for outputting a negative DC voltage, a half-wave rectification circuit for outputting a half-wave rectified signal, and a voltage amplifier, amplifies the half-wave rectified signal by the voltage amplifier, adds the negative DC voltage to the amplified half-wave rectified signal, and applies the sum signal to the electroabsorption optical modulator as the pulse modulation signal, and a dispersion decreasing fiber for receiving optical pulses output from the electroabsorption optical modulator at one end, and outputting the incoming optical pulses as final optical pulses from the other end.
In the optical pulse generation device with this arrangement, the dispersion decreasing fiber is further connected to the optical pulse generation device according to invention (2) above.
The dispersion decreasing fiber has characteristics in which the wavelength dispersion of light input from one end decreases with increasing distance if a predetermined wavelength condition and an optical fiber length condition (soliton condition) are satisfied, as is well known.
As a result, the pulse width Ts of optical pulses output from the other end is further shortened.
An optical sampling pulse generation apparatus according to invention (7) has the arrangements of the optical pulse generation devices according to inventions (5), (6) and their features, and also has an arrangement unique to an optical sampling pulse generation apparatus serving as a light sampling waveform measurement apparatus and their features.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.